Hi Miklos & Amir, Could you please take a look at this? It behaves different between the latest kernel and an old one, e.g. 4.9. Thanks, Joseph On 19/10/28 14:21, JeffleXu wrote: > Hi, Miklos, > > I noticed a performance regression of reading/writing files in mergeddir caused by commit a6518f73e60e5044656d1ba587e7463479a9381a (vfs: don't open real), using unixbench fstime. > > > Reproduce Steps: > > 1. cd /mnt/lower/ && git clone https://github.com/kdlucas/byte-unixbench.git > > 2. mount -t overlay overlay -olowerdir=/mnt/lower,upperdir=/mnt/upper,workdir=/mnt/work /mnt/merge > > 3. cd /mnt/merge/byte-unixbench/UnixBench && ./Run -c 1 -i 1 fstime > > > The score is 2870 before applying the patch, while it is 1780 after applying the patch, causing a 40% performance regression. > > The testcase repeatedly reads 1024 bytes from one file and writes the readed data into another file, while both these two files > > are created under /mnt/merge/tmp. I have testsed the latest kernel 5.4.0-rc4+, same results. > > > The perf shows that there's extra one call of file_remove_privs(), override_creds() and revert_creds() every write() syscall, > > among which file_remove_privs() is pretty expensive. > > > - perf data before applying the patch. > > ``` > > - 53.00% 0.93% fstime [kernel.kallsyms] [k] __vfs_write > - 52.08% __vfs_write > - 51.94% ext4_file_write_iter > + 48.89% __generic_file_write_iter > 0.83% down_write_trylock > 0.79% up_write > > ``` > > > - perf data after applying the patch. > > ``` > > + 94.88% 0.00% fstime [kernel.kallsyms] [k] entry_SYSCALL_64_after_hwframe > + 94.88% 4.67% fstime [kernel.kallsyms] [k] do_syscall_64 > + 66.08% 1.60% fstime libc-2.17.so [.] __GI___libc_write > + 62.37% 0.23% fstime [kernel.kallsyms] [k] ksys_write > + 61.74% 0.62% fstime [kernel.kallsyms] [k] vfs_write > - 60.10% 0.49% fstime [kernel.kallsyms] [k] __vfs_write > - 59.61% __vfs_write > - 59.56% ovl_write_iter > - 33.81% do_iter_write > - 32.50% do_iter_readv_writev > + ext4_file_write_iter > + 19.15% file_remove_privs > 2.15% revert_creds > 2.02% override_creds > 0.64% down_write > 0.63% up_write > > ``` > > > Regards. > > Jeffle >
